This article is about naturally occurring objects. For astronomical objects of the Solar System, seeList of Solar System objects. For artificial objects, seeSatellite.
"Celestial object" and "Celestial body" redirect here. For the subtle body postulated in philosophy, seeBody of light. For other uses, seeCelestial.
Anastronomical object,celestial object,stellar object orheavenly body is a naturally occurringphysical entity, association, or structure that exists within theobservable universe.[1] Inastronomy, the termsobject andbody are often used interchangeably. However, anastronomical body orcelestial body is a single, tightly bound, contiguous entity, while an astronomical or celestialobject is a complex, less cohesively bound structure, which may consist of multiple bodies or even other objects with substructures.
Astronomical objects such asstars,planets,nebulae,asteroids andcomets have been observed for thousands of years, although early cultures thought of these bodies asdeities. These early cultures found the movements of the bodies very important as they used these objects to help navigate over long distances, tell between the seasons, and to determine when to plant crops. During theMiddle Ages, cultures began to study the movements of these bodies more closely. Several astronomers of the Middle East began to make detailed descriptions of stars and nebulae, and would make more accurate calendars based on the movements of these stars and planets. In Europe, astronomers focused more on devices to help study the celestial objects and creating textbooks, guides, anduniversities to teach people more about astronomy.
During theScientific Revolution, in 1543,Nicolaus Copernicus'sheliocentric model was published. This model described theEarth, along with all of the other planets as being astronomical bodies which orbited theSun located in the center of theSolar System.Johannes Kepler discoveredKepler's laws of planetary motion, which are properties of the orbits that the astronomical bodies shared; this was used to improve the heliocentric model. In 1584,Giordano Bruno proposed that all distant stars are their own suns, being the first in centuries to suggest this idea.Galileo Galilei was one of the first astronomers to use telescopes to observe the sky, in 1610 he observed the four largest moons ofJupiter, now named theGalilean moons. Galileo also made observations of the phases ofVenus, craters on theMoon, andsunspots on the Sun. AstronomerEdmond Halley was able to successfully predict the return ofHalley's Comet, which now bears his name, in 1758. In 1781,Sir William Herschel discovered the new planetUranus, being the first discovered planet not visible by the naked eye.
In the 19th and 20th centuries, new technologies and scientific innovations allowed scientists to greatly expand their understanding of astronomy and astronomical objects. Larger telescopes and observatories began to be built and scientists began to print images of the Moon and other celestial bodies on photographic plates. Newwavelengths of light unseen by the human eye were discovered, and new telescopes were made that made it possible to see astronomical objects in other wavelengths of light.Joseph von Fraunhofer andAngelo Secchi pioneered the field ofspectroscopy, which allowed them to observe the composition of stars and nebulae, and many astronomers were able to determine the masses of binary stars based on theirorbital elements. Computers began to be used to observe and study massive amounts of astronomical data on stars, and new technologies such as thephotoelectricphotometer allowed astronomers to accurately measure the color and luminosity of stars, which allowed them to predict their temperature and mass. In 1913, theHertzsprung–Russell diagram was developed by astronomersEjnar Hertzsprung andHenry Norris Russell independently of each other, which plotted stars based on their luminosity and color and allowed astronomers to easily examine stars. It was found that stars commonly fell on a band of stars called themain-sequence stars on the diagram. A refined scheme forstellar classification was published in 1943 byWilliam Wilson Morgan andPhilip Childs Keenan based on the Hertzsprung–Russell diagram. Astronomers also began debating whether other galaxies existed beyond theMilky Way, these debates ended whenEdwin Hubble identified theAndromeda nebula as a different galaxy, along with many others far from the Milky Way.
Theuniverse can be viewed as having a hierarchical structure.[2] At the largest scales, the fundamental component of assembly is thegalaxy. Galaxies are organized intogroups and clusters, often within largersuperclusters, that are strung along greatfilaments between nearly emptyvoids, forming a web that spans the observable universe.[3]
The constituents of a galaxy are formed out of gaseous matter that assembles through gravitational self-attraction in a hierarchical manner. At this level, the resulting fundamental components are the stars, which are typically assembled in clusters from the various condensing nebulae.[6] The great variety of stellar forms are determined almost entirely by the mass, composition and evolutionary state of these stars. Stars may be found in multi-star systems that orbit about each other in a hierarchical organization. A planetary system and various minor objects such as asteroids, comets and debris, can form in a hierarchical process of accretion from theprotoplanetary disks that surround newly formed stars.
The various distinctive types of stars are shown by theHertzsprung–Russell diagram (H–R diagram)—a plot of absolute stellar luminosity versus surface temperature. Each star follows anevolutionary track across this diagram. If this track takes the star through a region containing anintrinsic variable type, then its physical properties can cause it to become avariable star. An example of this is theinstability strip, a region of the H-R diagram that includesDelta Scuti,RR Lyrae andCepheid variables.[7] The evolving star may eject some portion of its atmosphere to form a nebula, either steadily to form aplanetary nebula or in asupernova explosion that leaves aremnant. Depending on the initial mass of the star and the presence or absence of a companion, a star may spend the last part of its life as acompact object; either awhite dwarf,neutron star, orblack hole.
Any natural Sun-orbiting body that has not reached hydrostatic equilibrium is classified by the IAU as asmall Solar System body (SSSB). These come in many non-spherical shapes which are lumpy masses accreted haphazardly by in-falling dust and rock; not enough mass falls in to generate the heat needed to complete the rounding. Some SSSBs are just collections of relatively small rocks that are weakly held next to each other by gravity but are not actually fused into a single bigbedrock. Some larger SSSBs are nearly round but have not reached hydrostatic equilibrium. The small Solar System body4 Vesta is large enough to have undergone at least partial planetary differentiation.
Stars like the Sun are also spheroidal due to gravity's effects on theirplasma, which is a free-flowingfluid. Ongoingstellar fusion is a much greater source of heat for stars compared to the initial heat released during their formation.
Logarithmic representation of the observable universe with the notable astronomical objects known today. From down to up the celestial bodies are arranged according to their proximity to the Earth.
Infographic listing 210 notable astronomical objects marked on a central logarithmic map of the observable universe. A small view and some distinguishing features for each astronomical object are included.
^Task Group on Astronomical Designations from IAU Commission 5 (April 2008)."Naming Astronomical Objects". International Astronomical Union (IAU).Archived from the original on 2 August 2010. Retrieved4 July 2010.{{cite web}}: CS1 maint: numeric names: authors list (link)
^Elmegreen, Bruce G. (January 2010). "The nature and nurture of star clusters".Star clusters: basic galactic building blocks throughout time and space, Proceedings of the International Astronomical Union, IAU Symposium. Vol. 266. pp. 3–13.arXiv:0910.4638.Bibcode:2010IAUS..266....3E.doi:10.1017/S1743921309990809.
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